Oki LD670 User Manual page 170

2. Command trace mode
Quickly press the FEED switch (up to one second), and then press and hold
down the FEED switch for at least one second to accept the selection.
In command trace mode, all data sent from the host computer to the printer
is printed as command names. The printed data can be used to confirm that
correct control codes have been sent to the printer by a created program.
To reset this mode, turn off the power switch once.
Example of command trace output
168